Method and apparatus for forming a focused monoenergetic ion beam



T. M. DICKINSON METHOD AND APPARATUS FOR FORMING A FOCUSED June 23, 1970MONOENERGETIC ION BEAM Filed NOV. 4, 1968 6 8 0 Z 6 2 3 2 2 J M mm 3,1A, d 2 4 M M m 8 B i 4 0 W 2 m OW M m l l 0.. 5+ 2 Di 0 4 5 m\ I [All]F C 6 0 RY 4 7 M (/4 4 6 x P 4 4 2 mw a 7 2 W\\\\\\ F ILAM ENT SUPPLYELECTRON aawt NEXPAND/NG' VAPOR mm, TOR:

THEODORE M. DICKINSON,

H/S A TJTORNE Y United States Patent US. Cl. 313-63 7 Claims ABSTRACT OFTHE DISCLOSURE A method and apparatus are described for producing afocused beam of monoenergetic ions from an electrically conductingsource in the solid or liquid state by electron bombardment of thesource to produce an ionized vapor cloud from which cloud the generatedions are extracted by a remotely positioned negative acceleratingelectrode. Focusing electrodes are provided proximate the ion source tofocus the generated ion beam back along the axis of the electron beamand a common electrode is employed both to accelerate electrons to theion source and to accelerate ions in the reverse direction. By focusingthe beam to a relatively small diameter spot on the source, the ionizedvapor cloud is small relative to the electrode spacing and the ion beamthus formed is essentially monoenergetic.

This invention relates to a method and an apparatus for the generationof a focused monoenergetic ion beam and in particular to the formationof a focused monoenergetic ion beam by positioning ion focusing meansproximate a dense ion source while disposing a negative potential ionaccelerating electrode at a remote location relative to the source.

Among techniques presently utilized in the formation of semiconductivedevices are the ion implantation of a dopant into a semiconductivewafer, to produce asymmetrically conducting junctions at discretelocations in the wafer. Desirably the ion beam employed for implantationhas a minimum divergence to permit focusing of the beam upon selectedareas of the wafer and the depth of ion penetration into the wafer is afunction of the energy level of the ions impinging upon the wafersurface.

customarily, ion beams heretofore have been generated by the electronbeam irradiation of solid or gaseous mediums to form an ionized cloudfrom which cloud the generated ions are extracted utilizing an ionaccelerating electrode. However, when a gaseous medium serves as asource for a coaxial ion beam, ions are accelerated through a potentialgradient in the cathode fall region between the plasma and the electronemitting cathode and, because the energy level of each ion isproportional to the potential gradient through which the ion isaccelerated, the coaxial ion beams are characteristically broad spectrumenergy beams. In those circumstances where ion generation has beenachieved by electron beam irradiation of solids, e.g. those materialsfor which gaseous dispersions are difficult to obtain, the ionaccelerating electrode generally has been positioned proximate thematerial to draw a majority of the generated ions from the source vapor.In this case also, substantial quantities of the generated ions areaccelerated through Widely differing voltage gradients and the ion beamis characterized by a broad spectrum energy level.

An additional problem encountered during the formation of an ion beam ata coaxial attitude relative to the ion generating electron beam is thetendency for the ion beam to become divergent due to the action of theelectron beam focusing fields upon the ions. Similarly, when extensiveion focusing means are positioned between the ice ion source and theelectron beam source, the fine focusing of the electron beam upon thesurface of the ion source material is impaired thereby adverselyaffecting the formation of a monoenergetic ion beam.

It is therefore an object of this invention to provide an ion generatorcapable of producing a monoenergetic ion beam from a nongaseous source.

It is also an object of this invention to provide an ion generatorhaving an ion focusing system producing an ion beam with a minimumdivergence.

It is a further object of this invention to provide a novel method ofsimply forming a monoenergetic ion beam.

These and other objects of this invention generally are obtained byimpinging an electron beam upon a solid source in sufiicient intensityto vaporize and ionize a portion of the source, drawing the generatedions from the source vapor utilizing an initial ion accelerating fieldmeans disposed at a location remote from the source surface, e.g. at aspan from the source preferably at least 50-fold the diameter of theelectron beam upon the source, and focusing the generated ionsintermediate the source and the initial accelerating field means to forma converging ion beam at the accelerating field means. Thus an iongenerator in accordance with this invention would include means forgenerating an electron beam and means for focusing the electron beamupon the surface of an electrically conductive source material tovaporize a portion of the source. The vaporized material from the sourcethen interacts with the electron beam to form positively charged ionsand means are provided for forming an ion accelerating field to draw thegenerated ions from the source vapor in a substantially monoenergeticstream. Ion focusing means also are provided intermediate the ion sourceand the accelerating field means to form a convergent ion beam at theaccelerating field means.

In a preferred embodiment of this invention, the ion beam and electronbeam are coaxial and a common centrally apertured electrode ispositioned intermediate the electron source and the ion source to serveas an accelerating electrode for ions and electrons passingtherethrough. By disposing the ion focusing apparatus in the ionaccclerating region to form a convergent ion beam at the aperture in theaccelerating electrode, the ions have a high energy level entering theelectron beam focusing region of the ion generator and a minimumdivergence is produced as the high energy ion beam travels therethrough.

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, together withfurther objects and advantages thereof may best be understood byreference to the following description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is a simplified sectional view of an ion generator constructed inaccordance with this invention, and

FIG. 2 is an enlarged pictorial illustration of the vapor cloud formedby electron beam impingement upon the ion source. I

An ion generator 10 constructed in accordance with this invention isdepicted in FIG. 1 and generally includes a filamentary cathode 12 forthe generation of an electron beam which beam is focused upon solidmetallic source 14 in sufficient intensity to vaporize a portion of thesource thereby producing a high density vapor region 16 immediatelyadjacent the impinged surface of the source. Electron beam penetrationinto the vapor region produces an ionization of the vapor and the ionsthus formed are withdrawn by accelerating electrode 18 centrallysupported by insulating rods 20 intermediate cathode 12 and source 14 ata span from the source at least 50-fold the electron beam diameter uponthe source to produce a monoenergetic ion beam (as will be more fullyexplained hereinafter). The entire structure is enclosed by a top plate22 and a cylindrical sidewall 24 fixedly secured to the top plate bysuitable means, such as screws 26 threadedly engaged in threaded bores28 in flange 30, with air leakage into the chamber being prevented bygaskets 31. The lower end of the generator is secured to an ionutilization apparatus 32 e.g., an ion accelerator, and the interior ofthe ion generator is communicated with the vacuum system (not shown) ofapparatus 32 to produce the desired subatmospheric pressure, e.g. X10torr. for optimum functioning of the electron source. In general, avacuum less than torr. is required for a spherical expansion of thevapor boiled off the surface of source 14 and a sharp decrease in vapordensity with distance from the electron beam spot on the source. Thecontinuous exhaust of ion generator 10 by the vacuum system ofutilization apparatus 32 also inhibits the formation of a glow dischargethroughout ion focusing chamber 33 of the ion generator as a result ofthe high potentials applied to the electrodes.

Although any nongaseous electron beam source can be employed forirradiation of source 14, an electron gun having an annular filamentarycathode 12 and spherically shaped electron accelerating anode 18 ispreferred because of the ability of the gun to inherently generate wellfocused electron beam at high power levels while providing a cathodeaperture for the nondestructive passage of the generated ion beamtherethrough. An electron focusing electrode 36 extends outwardly fromcathode 12 in a generally spherical arc approximately concentric withthe arcuate face of accelerating electrode 18 proximate the cathode tofocus electrons emitted from the cathode through an aperture 38 withinthe accelerating electrode. Although electron focusing electrode 36 ispreferably concentric relative to the accelerating electrode arcproximate the cathode to produce radial lines of force in the areabetween cathode 12 and accelerating electrode 18, it is to be realizedthat minor modifications in accordance with well known techniques may beemployed in the focusing electrode geometry, e.g. the focusing electrodemay desirably have a smaller radius curvature or may have a shape whichis conical rather than spherical dependent upon desired operatingconditions, to compensate for space charge effects and to effect aconvergence of the generated electrons in a small diameter upon source14. Similarly, when a high ion density from the generator is desired,cathode 12 can be designed as a spherical arc having a central apertureto permit the passage of generated ions into utilization apparatus 32.

A suitable power supply 40 is serially connected to the filamentarycathode to provide energization therefor and one terminal of powersupply 40 is connected to electron focusing electrode 36 to maintain thepotential of the focusing electrode constant relative to the cathodepotential. Alternatively, an additional power supply may be provided toproduce a potential difference between electron source 12 and focusingelectrode 36 to provide a fine focus control to correct formanufacturing tolerances. The potential of accelerating electrode 18 isfixed at a positive value relative to the cathode by a DC. power supply42 while a second DC. power supply 44 connected in series additiverelationship for power supply 42 serves to maintain source 14 positiverelative to both accelerating elec trode 18 and cathode 12.

Accelerating electrode 18 fixedly positioned by insulating rods 20approximately midway between cathode 12 and source 14 can be anyconductive metal with molybdenum advantageously being employed when highpotentials are employed to accelerate the electron beam from cathode 12.

To assure a radial force focusing the generated electrons and formedions into an axial position passing through accelerating electrode 18,the center portion of accelerating electrode 18 is spherically shapedwith the spherical are 46 proximate source 14 being concentric with ionfocusing electrode 34 while the spherical are 48 r of the acceleratingelectrode face proximate cathode 12 is concentric relative to thespherical arc of electron fousing electrode 36. In general the sidewallsof aperture 38 Within accelerating electrode 18 are parallel relative tothe axis of the electron beam and the aperture has a diameter permittingpassage of the electron beam therethrough without intercept.

Source 14 from which the ion stream is produced is situated at thegeometric center of ion focusing electrode 34 which electrode has thegeneral configuration of the electron focusing electrode 36 previouslydescribed, i.e. a spherical arc symmetrically disposed relative to theare formed by electron focusing electrode 36. The source itself ismounted upon a plate 50 fixedly secured to and electrically joined withthe ion focusing electrode 34 surrounding the source by screws 48 topermit the convenient replacement or exchange of sources. Because of thegenerally symmetrical arrangement of ion focusing electrode 34 andelectron focusing electrode 36, ions formed by electron beam irradiationof source 14 are focused by electrode 34 along an axis coincident withthe electron beam axis and the ion beam sequentially passes through thecentral apertures within accelerating electrode 18, filamentary cathode12 and an exit aperture 70 in plate 72 mounted on the face of theelectron focusing electrode remote from source 14. With source 14 lyingat the geometric center of ion focusing electrode 34, ions formed byelectron beam irradiation of the source are focused immediately uponformation into a beam coaxial relative to the electron beam and few ofthe generated ions tend to diffuse throughout the chamber. Preferably,the arc of electrode 34 extends over a distance of at least 30% of thelinear span between source 14 and electrode 18 to maximize the focusingof the ions over a substantial portion of their accelerated travel. Ingeneral, ion focusing electrode 34 directs the ions produced byirradiation of source 14 into a slightly convergent beam as thegenerated ions pass through aperture 38 thereby enabling the ion beam tobe readily refocused to a fine spot or deflected with a minimumdispersion in the utilization apparatus while exit aperture 70 issituated at the focal point of ion focusing electrode 34 and desirablyis of a diameter less than 3 fold the diameter of the electron beam uponsource 14 to intercept ions of any undesired energy level.

Accelerating eletrode 18 is positioned at a distance at least SO-foldthe diameter of the electron beam upon source 14 to produce amonoenergetic ion beam. Because the accelerating electrode ideally ispositioned midway between the cathode and source to serve as anaccelerating electrode both for ions and for electrons passingtherethrough, the span between the focusing electrode and thefilamentary cathode 12 preferably is at least SO-fold the diameter ofthe electron beam upon source 14.

Varying operating conditions, eg, the potential gradient between source14 and accelerating electrode 18 and the diameter of the beam uponsource 14, can require a slight alteration in the geometry of the ionfocusing electrode, e.g. the outer extremities of the focusing electrodemay require a radius of curvature slightly shorter than a spherical arc,to assure that ions passing through aperture 38 are slightly convergent.Thus as the ions pass into the region between cathode 12 andaccelerating electrode 18, the divergence produced by electron focus ingelectrode 36 tends to compensate the slight con vergence in the beam toproduce a generally parallel beam exiting through the center ofcathode12.

Source 14 is any solid or liquid electrically conductive material whichis desirably ionized for a particular purpose. While the source can bein ingot form, a foil source supported upon a block of a material havinga substantially higher evaporation temperature and a relatively lowerthermal conductivity generally is preferred to limit heat loss throughthe source during operation. To inhibit contamination of the ion stream,e.g. by solid or gaseous impurities in the source material, the foilsource should be a high purity film preferably formed by vacuum melttechniques. Because the rod supports for accelerating electrode 18provide an open structure, any occluded gases are exhausted rapidly bythe vacuum system of the utilization apparatus.

As is illustrated more clearly in FIG. 2, the electron beam produced bycathode 12 is focused upon source 14 in an intensity to produce avaporization of a portion of the source and the vapor diffuses outwardlyin a generally hemispherically shaped vapor region 16 within the vacuumoperating conditions of ion generator 10. Hemispheric vapor region 16characteristically has a radius approximately equal to the radius of theelectron beam upon the surface of source 14 with the vapor density ofthe region being dependent upon the bombarding electron beam density andthe vapor pressure of the evaporating source. Because the density of thevapor formed by the evaporating source decreases in the vacuumenvironment of the system as the square of the distance from the source,at a distance approximately electron beam radii from the surface ofsource 14, the density of the vapor cloud (and hence the ion formationrate) is reduced to approximately 4%. The total number of ions formedalong the path of the electron beam is obtained by integrating thedensity as a function of distance with the result that the total ionsgenerated beyond any radius decreases as the first power of radius. Thus80% of the electron beam formed ions are located within an arc 62 havinga radius approximately 5 fold the radius of the electron beam upon thetarget surface. By positioning the most proximate ion acceleratingelectrode 18 at a minimum distance of 50 fold the electron beam diameterupon source 14, the ions formed within the relatively high density vaporhemisphere enclosed by are 62 are accelerated through a voltage gradientwhich is at least 95% of the maximum. 80% of the ions generatedtherefore have an energy level not deviating by more than approximately2.5% from the medium value. The remaining 20% of the ions of lowerenergy formed in the path of the electron beam beyond are 62 are formedoff axis because of the increasing width of the electron beam and areforward of the focal plane of focusing electrode 34 with the result thatthey are not focused on the exit aperture 70 and do not emerge from thestructure. Hence 100% of the emergent beam consists of those ions formedWithin 5 electron beam radii from the surface of source 14 and have anenergy spread of :2.5% from the medium value.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An ion generator for producing a focused beam of monoenergetic ionscomprising means for generating an electron beam, a source ofelectrically conductive material, means for focusing said electron beamupon the surface of said source material to vaporize and ionize aportion of said source material, means for forming an initial ionaccelerating field to draw ions from said source vapor to saidaccelerating field means, and means for focusing said ions intermediatesaid source and said accelerating field means to form a converging ionbeam at said initial ion accelerating field means, said ion focusingmeans and said electron focusing means being symmetrically disposed toform said ion and said electron beams along a coaxial path.

2. An ion generator according to claim 1 wherein said initial ionaccelerating field is at a span from said electron impinged sourcesurface at least 50-fold the diameter of said beam upon said sourcesurface.

3. An ion generator according to claim 2 further including a platepositioned at the focal plane of said ion focusing means, said platehaving an aperture coaxial with said ion beam and being of a diameterless than 3- fold the diameter of said electron beam upon said source.

4. An ion generator for producing a focused beam of monoenergetic ionscomprising a cathode, a metallic target, means for energizing saidcathode relative to said target to generate an electron beamtherebetween, means for focusing said electron beam upon said target ata sufi'icient intensity to vaporize and ionize a portion of said metaltarget, and apertured electrode means for accelerating said ionsgenerated by said electron beam, said apertured electrode means beingcircumferentially disposed about said electron beam and situated fromsaid target at a span at least 50-fold the diameter of said electronbeam upon said target to draw ions from said target vapor in asubstantially monoenergetic stream.

5. An ion generator according to claim 4 further including an ionfocusing electrode having the physical configuration of a spherical arc,said target being situated at the geometric center of said electrode.

6. A method of forming a focused beam of monoenergetic ions comprisingimpinging an electron beam upon a metallic source at a sufficientintensity to vaporize a portion of said source, said vaporized sourceinteracting with said electron beam to form metallic ions, forming anegative potential field at a span from said metallic source surface atleast 50-fold the diameter of said electron beam upon said sourcesurface to draw ions from said source vapor to said negative potentialfield in a substantially monoenergetic stream, and focusing said ionsintermediate said metallic source and said negative potential field toproduce a slightly converging ion beam at said negative potential field.

7. A method of forming an electron beam according to claim 6 comprisingintercepting ions of an undesired energy level upon an apertured platepositioned in the focal plane of said ion focusing means.

References Cited UNITED STATES PATENTS 1/1953 Washburn et al. 313-230 X1/1966 Stevens et al. 25041.9 X

